The manufacture of compression molded composites from a mixture of wood chips or fibers and a polyisocyanate binder is well known. Isocyanate binders are excellent adhesives for the wood chips or fibers, but they require the use of high press temperatures during the manufacture of composite boards from these lignocellulosic particulate substrates. Press temperatures of greater than 400 F, often as high as 430 F or even higher, are often needed in order to achieve an effective cure of the adhesive in a reasonable period of time. There is a trade off between press temperature and residence time in the press. Lower press temperatures generally equate to longer residence times. Longer residence times are undesirable in industry because these reduce the throughput of the manufacturing process. High press temperatures require more energy and often cause undesirable amounts of VOC (volatile organic compounds) emissions from the substrate during pressing. In addition, the high temperatures often cause thermal damage to the composite boards, and, in some circumstances, even fires in the press. The high heat requirements tend to drive the moisture content of the end product (board) to undesirably low levels.
The types of polyisocyanate adhesives used in industrial practice are almost invariably isocyanates of the MDI series, usually polymeric MDI (which refers to various mixtures of diphenylmethane diisocyanates and higher molecular weight polymethylene polyphenyl polyisocyanate oligomers). These polyisocyanate adhesives represent an improvement over older types of adhesives, most notably phenol-formaldehyde (PF) type adhesives, in terms of the quality of the boards produced. Lignocellulosic composites produced using MDI adhesives are generally more resistant to moisture damage than similar boards made with PF resins. An additional advantage of MDI adhesives, which has made them increasingly attractive in the OSB (oriented strand board) industry, is greater moisture tolerance in the substrate. MDI adhesives, as opposed to PF resins, can often be used on wood substrates that have higher moisture content. PF resins require very high pressing temperatures for industrially practical press residence times. The widely used class of MDI polyisocyanates work best with wood substrates (chips and/or fibers, often referred to in the industry as “furnish”) that have been pre-dried to a moisture content in the range of about 2 to 6% by weight. This still requires some pre-drying of the furnish, although generally not as much as for PF adhesives. Pre-drying uses large amounts of energy and requires an added step in the production process. It also requires additional apparatus and introduces a new class of hazard—dryer fires. It would be highly desirable in industrial practice to avoid the pre-drying step altogether, or at least to achieve further reductions in the amount of pre-drying that is needed.
Therefore, there is a strong need in the wood composites industry, particularly in the manufacture of pressed particulate wood composites such as oriented strand board (OSB), for adhesives that can be used on higher moisture content furnishes and will cure at lower press temperatures without a decrease in press throughput. It would be desirable to be able to use press temperatures less than 400 F, preferably 350 F or lower, without increasing press residence time. It would also be desirable to be able to use furnish with a moisture content greater than 6% by weight. An adhesive that can successfully meet all these needs has the potential to significantly reduce product costs by reducing energy consumption. Additional benefits include improved product quality, reduced emissions, and improved plant safety.